VMs with PVSCSI Controllers could have 256 disks, but the command Add-NewDisk limits it to 64 disks.
An workaround is to specify an disk controller with the parameter "-controller"
"New-HardDisk" should verify if the VM contoller is PVSCSI, to considerer the limit of 64 disks per controller.
Shipped with PowerCLI 12.5